1. Field of the Invention
The present invention relates to a hybrid sensor including an electrostatic capacitance sensor, which detects a nearby object utilizing a variation in electrostatic capacitance, and a contact pressure sensor which detects a load pressure. More particularly, the present invention relates to a hybrid sensor which is fabricated by stacking a plurality of elastic or flexible sheets each of which is provided with electrode patterns.
2. Description of Related Art
A hybrid sensor of related art having an electrostatic capacitance sensor and a contact pressure sensor as well as a system utilizing the hybrid sensor for detecting a human body set on a seat of an automobile is disclosed in Japanese Unexamined Patent Application, First Publication, No. 2002-326554, assigned to the same applicant as that of the present application. The hybrid sensor 6 is, as shown in FIG. 8, formed by stacking upper and lower flexible sheets 11 and 12 made of insulating resin film such as polyester film and an insulating sheet 13 positioned therebetween. The flexible sheets 11 and 12 are provided with switching electrode patterns 14 and 15, respectively, on a lower surface 11a of the sheet 11 and an upper surface 12a of the sheet 12 which face each other. The electrode patterns 14 and 15 are formed by screen printing in which a suitable conducting material such as silver paste is deposited. Each of the electrode patterns 14 and 15 includes a plurality of land portions 14a and 15a which are used as switching contacts 16 arranged in a matrix pattern and lead portions 14b and 15b which electrically connect the land portions 14a and 15a to a detecting circuit (not shown). The insulating sheet 13 has a plurality of openings 17 at positions corresponding to the land portions 14a and 15a. Each of the openings 17 has a diameter sufficient to enable one of the land portions 14a to contact the associated one of the land portions 15a. When a pressure load is applied, one or some of the land portions 14a of the upper flexible sheet 11 contact the associated one or some of the land portions 15a of the lower flexible sheet 12 through the openings 17 to make an electrical connection. The hybrid sensor 6 performs the function of the contact pressure sensor by the structural elements described above.
The function of the electrostatic capacitor sensor in the hybrid sensor 6 is achieved by a pair of detecting electrode patterns 7a and 7b formed on the upper flexible sheet 11 on the surface 11a facing the insulating sheet 13. The detecting electrode patterns 7a and 7b are insulated from the switching electrode patterns 14. The electrostatic capacitor sensor thus constructed detects a nearby object without contacting thereto according to a variation in electrostatic capacitance obtained from the detecting electrode patterns 7a and 7b. 
According to the above structure, the manufacturing cost for the hybrid sensor can be reduced since the contact pressure sensor and the electrostatic capacity sensor are formed on the same flexible sheets.
However, the hybrid sensor described above inevitably has an insufficient sensitivity to detect the nearby object when the object is relatively far from the sensor. This is because the detecting electrode patterns for the electrostatic capacitance sensor are provided on a relatively small area of the flexible sheet.